Tunable laser with directional coupler

ABSTRACT

A tunable laser has a first mirror, a second mirror, a gain medium, and a directional coupler. The first mirror and the second mirror form an optical resonator. The gain medium and the directional coupler are, at least partially, in an optical path of the optical resonator. The first mirror and the second mirror comprise binary super gratings. Both the first mirror and the second mirror have high reflectivity. The directional coupler provides an output coupler for the tunable laser.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/949,937, filed on Mar. 7, 2014, entitled “DIRECTIONALSEMICONDUCTOR WAVEGUIDE COUPLER,” the disclosure of which isincorporated by reference in its entirety for all purposes. Thisapplication further claims priority to U.S. Provisional PatentApplication No. 61/950,658, filed on Mar. 10, 2014, entitled “TUNABLELASER WITH DIRECTIONAL COUPLER,” the disclosure of which is incorporatedby reference in its entirety for all purposes.

The following two U.S. patent applications (including this one) arebeing filed concurrently, and the entire disclosure of the otherapplication is incorporated by reference into this application for allpurposes:

-   Application Ser. No. 14,642,415, filed Mar. 9, 2015, entitled    “DIRECTIONAL SEMICONDUCTOR WAVEGUIDE COUPLER”; and-   Application Ser. No. 14,642,443, filed Mar. 9, 2015, entitled    “TUNABLE LASER WITH DIRECTIONAL COUPLER”.

BACKGROUND OF THE INVENTION

Optical waveguiding elements convey light from one point to anotherthrough an optically transparent, elongated structure by modaltransmission, total internal reflection, and/or total reflectorization.An optical waveguide directs radiation in the visible, infrared, and/orultra-violet portions of the radiation spectrum by total internalreflection.

This application further relates to tunable lasers. More specifically,and without limitation, to tunable semiconductor lasers using binarysuper gratings.

BRIEF SUMMARY OF THE INVENTION

In some embodiments, a laser has a first binary super grating (BSG), asecond BSG, a gain medium, and a directional coupler. The first BSG andthe second BSG form an optical resonator. The gain medium and thedirectional coupler are, at least partially, in an optical path of theoptical resonator.

A directional coupler shifts a portion of electromagnetic radiation(e.g., light in the UV, visible, and/or IR spectrums) from a firstwaveguide to a second waveguide. Light travels in a waveguide from oneor more inputs to one or more outputs. Often, the first waveguide andthe second waveguide are parallel for a given length, and a certainpercentage of light from the first waveguide transitions to the secondwaveguide through sides of the waveguides instead of through the ends ofthe waveguides.

In some embodiments, a directional coupler for optically couplingwaveguides comprises a first input; a second input; a first output; asecond output; a shoulder disposed on substrate; a first ridge disposedon the shoulder; and a second ridge disposed on the shoulder. Theshoulder extends from the first input to the first output, from thesecond input to the second output, from the first input to the secondoutput, and from the second input to the first output. The shoulder amaterial. The first ridge extends from the first input to the firstoutput, wherein the first ridge comprises the material. The second ridgeextends from the second input to the second output, wherein: the secondridge comprises the material; and the second ridge is separate from thefirst ridge. In some embodiments, the directional coupler furthercomprises a first region where the shoulder tapers; a second regionwhere the first ridge and the second ridge taper; a third region wherethe first ridge and the second ridge taper in an opposite direction thanin the second region; and a fourth region where the shoulder tapers inan opposite direction than in the first region. In some embodiments, theshoulder tapers in the first region to increase a width of the shoulder,wherein the shoulder extends beyond an outside edge of the first ridgeand beyond an outside edge of the second ridge.

In some embodiments, a method for coupling thick-silicon waveguidesusing a directional coupler is disclosed. Light is guided into a firstinput of the directional coupler, wherein: the directional coupler has afirst ridge extending from the first input to a first output; and thedirectional coupler has a second ridge extending from a second input toa second output. Light is guided from the first input, to the firstridge, and then the first ridge, through a shoulder, to the secondridge. Light is guided from the second ridge to the second output.

In some embodiments, a tunable laser comprises: a first wavelengthselective element characterized by a first reflectance spectrum; asecond wavelength selective element characterized by a secondreflectance spectrum, wherein the first wavelength selective element andthe second wavelength selective element form an optical resonator; again medium between the first wavelength selective element and thesecond wavelength selective element; and a directional coupler betweenthe first wavelength selective element and the second wavelengthselective element, wherein the directional coupler provides an outputcoupler for the laser.

In some embodiments, a method of operating a tunable laser is disclosed.A first a first wavelength selective element is tuned. A secondwavelength selective element is tuned, wherein the first wavelengthselective element and the second wavelength selective element form anoptical resonator. Optical emission is from a gain medium disposedbetween the first wavelength selective element and the second wavelengthselective element. The optical emission is guided to pass through adirectional coupler. A first portion of the optical emission istransmitted out of the optical resonator using the directional coupler.And a second portion of the optical emission is transmitted to the firstwavelength selective element using the directional coupler. In someembodiments, the first wavelength selective element comprises a binarysuper grating (BSG) having a first number of super periods; the secondwavelength selective element comprises a BSG having a second number ofsuper periods; and the first number of super periods equals the secondnumber of super periods. In some embodiments, the first number of superperiods is not more than one, two, and/or three greater than the secondnumber of super periods.

Further areas of applicability of the present disclosure will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating various embodiments, are intended for purposes ofillustration only and are not intended to necessarily limit the scope ofthe disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a top view of an embodiment of a directional coupler.

FIG. 2 depicts a cross-sectional view of an embodiment of thedirectional coupler.

FIG. 3 depicts a top view of an embodiment of the directional couplershowing regions of the directional coupler.

FIG. 4 depicts a top view and cross-sectional views of an embodiment ofthe directional coupler.

FIG. 5 depicts a graph of coupling efficiency compared to coupling widthof an embodiment of the directional coupler.

FIG. 6 depicts a top view of an embodiment of a directional coupler,wherein the directional coupler does not have ridge tapers.

FIG. 7 depicts a top view of an embodiment of a directional coupler,wherein the directional coupler has a first ridge that is tapered and asecond taper that is not tapered.

FIG. 8 depicts a graph of coupling efficiency compared to couplinglength for an embodiment of the directional coupler.

FIG. 9 depicts a flowchart of a process for coupling thick siliconwaveguides.

FIG. 10 depicts a simplified diagram of an embodiment of a tunable laserwith a directional coupler.

FIG. 11 depicts a simplified diagram of another embodiment of a tunablelaser with a directional coupler.

FIG. 12 is a chart comparing normalized combined responses of a Y-branchlaser and an embodiment of a laser with a directional coupler.

FIG. 13 is a zoomed up portion of the chart in FIG. 12.

FIG. 14 depicts a flowchart of a process for using a laser with adirectional coupler.

FIG. 15 depicts a top view of an embodiment of a directional couplerhaving a shoulder that tapers wider than ridges.

In the appended figures, similar components and/or features may have thesame reference label. Further, various components of the same type maybe distinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. If only the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label.

DETAILED DESCRIPTION OF THE INVENTION

The ensuing description provides exemplary embodiment(s) only, and isnot intended to limit the scope, applicability, or configuration of thedisclosure. Rather, the ensuing description of the exemplaryembodiment(s) provides those skilled in the art with an enablingdescription. It is to be understood that various changes may be made inthe function and arrangement of elements without departing from thespirit and scope as set forth in the appended claims.

Some embodiments relate generally to directionally coupling two opticalwaveguides. More specifically, and without limitation, to directionallycoupling high-contrast, thick-silicon waveguides using tapers. Inthick-silicon, modes are more tightly confined (wherein thick silicon isgreater than 0.3, 0.5, or 0.7 μm thick and/or has an effectiverefractive index greater than or equal to 2.9, 3.0, or 3.2). Thus simplybrining two waveguides closer together for coupling in thick silicon isnot as efficient as in thin silicon because in thin silicon modes arenot as tightly confined. Further, separating two thick-siliconwaveguides by a narrow distance can require more stringent processingtolerances than if waveguides are further apart. Also, some embodimentsdescribe using a directional coupler in a tunable laser system.

FIG. 1 is a top view of an embodiment of a directional coupler 100. Thedirectional coupler 100 has first input 104-1, a second input 104-2, afirst output 108-1, and a second output 108-2. The first input 104-1 andthe second input 104-2 are separated by a first gap 112. The firstoutput 108-1 and the second output 108-2 are separated by a second gap116.

The directional coupler 100 is disposed on a substrate. The directionalcoupler 100 comprises a lower portion, sometimes referred to as acoupler shoulder 120. The directional coupler 100 comprises two upperportions, sometimes referred to as a first ridge 124-1 and a secondridge 124-2. The coupler shoulder 120 is disposed on the substrate. Theridges 124 are disposed on portions of the coupler shoulder 120 so thatthe coupler shoulder 120 is between the ridges 124 and the substrate.The first ridge 124-1 extends from the first input 104-1 to the firstoutput 108-1. The second ridge 124-2 extends from the second input 104-2to the second output 108-2.

The directional coupler 100 is symmetrical about a longitudinal axis128. The longitudinal axis 128 is substantially in a direction of beampropagation from the inputs 104 to the outputs 108. The directionalcoupler 100 has a coupler waist 132. The coupler waist 132 isequidistant between the inputs 104 and the outputs 108. The couplerwaist 132 has a cross section perpendicular (laterally) to thelongitudinal axis 128. The coupler waist 132 is the most narrow part ofthe directional coupler 100.

FIG. 2 is a cross-sectional view of an embodiment of the directionalcoupler 100 at the coupler waist 132. The coupler shoulder 120 isdisposed on a substrate 200. The substrate 200 extends laterally widerthan the directional coupler 100 and longer (longitudinally) than thedirectional coupler 100. The first ridge 124-1 and the second ridge124-2 are disposed on the coupler shoulder 120 and separated by a widthequal to the first gap 112. Though the coupler shoulder 120 and theridges 124 are shown with different shading, the coupler shoulder 120and the ridges 124 are formed from the same type of material (e.g.,crystalline silicon; by etching and/or deposition). The coupler shoulder120 and the ridges 124 form a core of the directional coupler 100. Insome embodiments, the coupler shoulder 120 and the ridges 124 arecovered with an upper cladding (e.g., SiO2, epoxy, and/or photoresist)having an index of refraction that is less than the core of thedirectional coupler 100 so that light (e.g., optical modes of a beam oflight) is confined within the core of the directional coupler 100. Insome embodiments, the coupler shoulder 120 and the ridges 124 are notcovered with an upper cladding but left exposed to air as a cladding.The substrate 200 comprises as material that has a refractive index lessthan the core of the directional coupler 100, which acts as a lowercladding. In some embodiments, the substrate 200 comprises a buriedoxide (BOX) layer of a silicon-on-insulator (SOI) wafer; the core of thedirectional coupler 100 is formed from a device layer of the SOI wafer;and a handle portion of the SOI wafer is below the BOX layer.

The coupler shoulder 120 has a shoulder height 204 (height measured in adirection away from the substrate 200). In some embodiments, theshoulder height 204 ranges from 0 to 2.5 μm (e.g., 0.0, 0.1, 0.5, 0.75,0.85, 0.95, 1.0, 1.2, 1.5, 2.0, or 2.5 μm). The coupler shoulder 120 hasa shoulder width 208 (width measured in a lateral direction compared tothe longitudinal axis 128 and orthogonal to height). The shoulder width208 varies along the longitudinal axis 128.

The ridges 124 have a ridge height 214. In some embodiments, the ridgeheight 214 ranges from 0.1 to 2.0 μm (e.g., 0.1, 0.2, 0.3, 0.45, 0.55,0.65, 0.75, 1.0, 1.5, or 2.0 μm). The ridges 124 each have a ridge width218. The ridge width 218 varies along the longitudinal axis 128. At thecoupler waist 132, the first ridge 124-1 is separated from the secondridge 124-2 by the first gap 112. In some embodiments, the first ridge124-1 is separated from the second ridge 124-2 by the first gap 112along the longitudinal axis 128.

Referring next to FIG. 3, the coupler 100 is shown having four regions:a first region 301, a second region 302, a third region 303, and afourth region 304. The first region 301 comprises the inputs 104. In thefirst region 301 at the inputs 104, the coupler shoulder 120 isseparated into two portions: a first portion of the coupler shoulder 120under the first ridge 124-1 and a second portion of the coupler shoulder120 under the second ridge 124-2. At the inputs 104, the first portionof the coupler shoulder 120 is separated from the second portion of thecoupler shoulder 120 by the first gap 112. In the first region 301, thecoupler shoulder 120 tapers inward in a longitudinal direction, towardthe longitudinal axis 128, until the first portion of the couplershoulder 120 merges with the second portion of the coupler shoulder 120.In the first region 301, the ridge width 218 remains constant in thelongitudinal direction.

The second region 302 is adjacent, longitudinally, to the first region301. In the second region 302, outside edges 308 of the coupler shoulder120 and the ridges 124 taper (narrow). In the second region 302, thefirst portion of the coupler shoulder 120 is merged with the secondportion of the coupler shoulder 120 (e.g., indistinguishable and/orcontiguous so that there is no gap between the first portion of thecoupler shoulder 120 and the second portion of the coupler shoulder120). Thus, in some embodiments, light is not evanescently coupled, butdirectly coupled in the directional coupler 100. In some embodiments,light is directly coupled instead of evanescently coupled to havegreater coupling efficiency, more flexible manufacturing tolerances,and/or reduce attenuation. The ridges 124 remain separated by the firstgap 112.

The third region 303 is adjacent, longitudinally, to the second region302. The third region 303 mirrors the second region 302. In the thirdregion 303, outside edges 308 of the coupler shoulder 120 and the ridges124 taper (widen). The first portion of the coupler shoulder 120 remainsmerged with the second portion of the coupler shoulder 120. The ridges124 remain separated by the first gap 112.

The fourth region 304 is adjacent, longitudinally, to the third region303. The fourth region 304 mirrors the first region 301. In the fourthregion 304, the coupler shoulder 120 splits and tapers outward, away thelongitudinal axis 128, until the first portion of the coupler shoulder120 is separated from the second portion of the coupler shoulder 120 bythe second gap 116. Outside edges 308 of the coupler shoulder 120 andthe ridges 124 remains constant. In the fourth region 304, the ridgewidth 218 remains constant in the longitudinal direction. The ridges 124are separated by the second gap 116.

FIG. 4 is a top view and cross-sectional views of an embodiment of thedirectional coupler 100. FIG. 4 comprises a first cross section 401, asecond cross section 402, a third cross section 403, and a fourth crosssection 404. The first cross section 401 is of the directional coupler100 at the inputs 104. The second cross section 402 is of thedirectional coupler 100 in the first region 301, with the first portion408-1 of the coupler shoulder 120 and the second portion 408-2 of thecoupler shoulder 120 tapering inward toward the longitudinal axis 128.The third cross section 403 is of the coupler waist 132. The fourthcross section 404 is of the outputs 108.

The first portion 408-1 has a first width 418-1. The second portion408-2 has a second width 418-2. A total width of the coupler shoulder120 at the first cross section 401 is equal to the first width 418-1,plus the width of the first gap 112, plus the second width 418-2. A netwidth of the coupler shoulder 120 at the first cross section 401 isequal to the first width 418-1 plus the second width 418-2. In thesecond cross section 402, though the total width of the coupler shoulder120 remains constant compared to the first cross section 401; the netwidth of the coupler shoulder 120 increases because the first portion408-1 and the second portion 408-2 are tapering inward, toward thelongitudinal axis 128. In the third cross section 403, the total widthof the coupler shoulder 120 is equal to the net width of the couplershoulder 120 because there is no gap between the first portion 408-1 andthe second portion 408-2. The total width of the coupler shoulder 120 inthe third cross section 403 is more narrow than the total cross sectionof the coupler shoulder 120 in the second cross section 402 because inthe second region 302, outside edges 308 tapered inward. Some numberdesignators are left off some features as to not unduly clutter thefigures. A person skilled in the art will recognize similar featureseven though there are not number designators.

FIG. 5 is a graph of simulated coupling efficiency compared to the ridgewidth 218 at the coupler waist 132. Coupling efficiency is an amount ofpower that enters the first input 104-1 and exits the second output108-2; or the amount of power that enters the second input 104-2 andexits the first output 108-1. As the ridge width 218 at the couplerwaist 132 narrows, coupling efficiency increases. Thus the directionalcoupler 100 can be designed for a given coupling efficiency by modifyingthe ridge width 218 at the coupler waist 132.

FIG. 2 is an example of an embodiment of the directional coupler 100with a ridge width 218 of 1.5 μm at the coupler waist 132. According tothe plot in FIG. 4, the directional coupler 100 has bout about 40%coupling efficiency from the first input 104-1 to the second output108-2 for light at a wavelength of 1525 nm. For a ridge width 218 of 1.9μm, the directional coupler 100 would have a coupling efficiency alittle lower than 20% for light having a wavelength of 1525 nm. Lighthaving a wavelength of 1575 nm has a few more percentage couplingefficiency than light at 1525 nm for a given ridge width 218 at thecoupler waist 132.

FIG. 6 is a top view of a second directional coupler 600, thedirectional coupler 100 also being referred to as a first directionalcoupler 100. The second directional coupler 600 is similar to the firstdirectional coupler 100 except that ridges 124 of the second directionalcoupler 600 do not taper in the second region 302 or the third region303. The coupler shoulder 120 also does not taper in the second region302 or the third region 303. Also shown is a coupling length 604. Thecoupling length 604 begins where the first portion 408-1 and the secondportion 408-2 of the coupler shoulder 120 merge and extends, in alongitudinal direction, to where the first portion 408-1 and the secondportion 408-2 of the coupler shoulder 120 split. In some embodiments,the second directional coupler 600 is used instead of the firstdirectional coupler 100 to reduce processing complexity and/or to reducecoupling efficiency.

FIG. 7 is a top view of third directional coupler 700. The thirddirectional coupler 700 is asymmetrical about the longitudinal axis 128.The first ridge 124-1 of the third directional coupler 700 tapers (aswell as portions of the coupler shoulder 120 under the first ridge),similarly to the first ridge 124-1 of the first directional coupler 100.The second ridge 124-2 of the third directional coupler 700 does nottaper, similarly to the second ridge 124-2 of the second directionalcoupler 600. In some embodiments, having asymmetrical ridges 124 is tohave asymmetrical coupling efficiencies. In some embodiments, the seconddirectional coupler 600, the third directional coupler 700, and/orvariations on the second directional coupler 600 and/or the thirddirectional coupler 700, are used in place of the first directionalcoupler 100, based on design constraints of a system (e.g., desiredcoupling efficiency).

FIG. 8 is graph of simulated coupling efficiency compared to ahalf-length of the coupling length 604 for both 1525 nm and 1575 nmlight. For example, if the coupling length 604 of the second directionalcoupler 600 is 40 μm, the coupling efficiency for 1525 nm light is about2% (½ length of 40 μm is 20 μm; and the graph shows about 2% couplingefficiency at 20 μm). The longer the coupling length 604, the greaterthe coupling efficiency. In some embodiments, tapering the ridges 124and having a coupler waist 132 that is smaller is preferred to increasecoupling efficiency and/or to reduce a footprint size of the directionalcoupler 100 on a chip. In some embodiments, the coupling length 604 isless than 120, 90, 80, and/or 60 μm.

FIG. 9 depicts a flowchart of a process 900 for coupling thick siliconwaveguides. The process 900 begins in step 904 where light is guidedinto the first input 104-1 of the directional coupler 100. In step 908,the light is then guided from the first input 104-1, through the firstridge 124-1, and from the first ridge 124-1 to the second ridge 124-2through the coupler shoulder 120. In step 912, the light is then guidedfrom the second ridge 124-2 to the second output 108-2.

In some embodiments, a purpose of the directional coupler 100 is tocouple a first TE mode from the first input 104-1 to the second output108-2. In some embodiments, a purpose of the directional coupler 100 isto couple the first TE mode from the first input 104-1 to the secondoutput 108-2 in a relatively short distance (e.g., less than 150, 120,90, 80, and/or 60 μm). In some embodiments, tapering the couplershoulder 120 enables adiabatic expansion and/or compression of confinedelectrometric radiation in the directional coupler 100, thusfacilitating coupling the first TE mode (and/or first TM mode) from thefirst input 104-1 to the second output 108-2. Further, embodiments ofthis invention are not limited to silicon waveguides, or evensemiconductor waveguides. In some embodiments, dielectric directionalcoupler, and/or metallic directional couplers, are used. For example,polymer waveguides and/or waveguides using aluminum oxide, tantalumoxide, titanium oxide, or other dielectric materials could be used. Insome embodiments, ridge tapers are not confined to the second region 302and the third region 303. For example, a ridge taper can start in thefirst region 301; a ridge taper can be made to continue from the thirdregion 303 into the fourth region 304; and/or a ridge taper can start inthe fourth region 304.

Additionally, there are many possible variations to the embodimentsshown. For example, FIGS. 5 and 8 were simulations for a thick silicon,high-contrast waveguides on a substrate 200 (e.g., SiO2 orsilicon-on-insulator wafer) with the coupler shoulder 120 and the ridges124 covered with a cladding (e.g., SiO2). In other variations, thedirectional coupler 100 is exposed to air or cladded with an epoxyand/or a photoresist. Additionally, in some embodiments, the ridge width218 ranges from 0.4 to 3.5 μm. In some embodiments, the inputs 104 havea geometry to match a corresponding waveguide (e.g., a ridge waveguideinstead of a rectangular waveguide).

Further, even though FIGS. 5 & 8 show plots of simulation results for agiven range, those plots can be extrapolated beyond the plotted ranges.Another example variation for coupling, beyond FIGS. 5 and 8, isincreasing or decreasing the first gap 112 and/or the second gap 116between the ridges 124. In some embodiments, the first gap 112 is equalto the second gap 116. Decreasing the first gap 112 and/or the secondgap 116 will result in increased coupling; increasing the first gap 112and/or the second gap 116 will result in decreased coupling. In someembodiments, the first gap 112 and/or the second gap 116 have a widthfrom 0.3 to 3.5 μm. For example, the first gap 112 has a width from 0.3to 3.5 μm, or from 0.5 to 1.5 μm (e.g., 0.3, 0.5, 0.75, 1.0, 1.25, 1.5,1.75, 2.0, 2.5, 3.0, or 3.5 μm). In some embodiments, a gap (e.g., >0.75μm) is used to provide for more favorable fabrication tolerance thantolerances required to make a gap that is less than 0.5 μm. For example,a fabrication tolerance for a 100 nm gap requires special controlmeasures (e.g., temperature control, tight lithographic tolerances,smooth and vertical etching, proper deposition of SiO2/cladding to avoidair gaps and voids, and/or tight process control to avoid stress/strainduring SiO2 cladding deposition). A larger gap (e.g., >0.75 μm)facilitates easier manufacturing of the directional coupler 100.Dimensions of the directional coupler 100 can also vary as a function ofwavelength.

Further variations to the directional coupler 100 include variations totapers. For example, instead of a linear taper, a quadratic, log, and/oradiabatic curved tapers can be used. Additionally, the first ridge 124-1can have different tapers than the second ridge 124-2. For example, thefirst ridge 124-1 may have a decreasing and increasing taper as shown inFIG. 1. But the second ridge 124-2 has only a decreasing taper in thesecond region 302 and then the width of the second ridge 124-2 remainsconstant in the third region 303 and the fourth region 304. In anotherexample, the second ridge 124-2 has a quadratic decreasing taper in thesecond region 302 and a linear increasing taper in the third region 303.Additionally, the first region 301, the second region 302, the thirdregion 303, and/or the fourth region 304 do not need to be contiguous intheir concatenation. For example, there could be a fifth region betweenthe second region 302 the third region 303.

FIG. 10 depicts a simplified diagram of an embodiment of a first lasersystem 1000-1 having a directional coupler 100. The first laser system1000-1 comprises a first mirror 1001, a second mirror 1002, a gainmedium 1004, and the directional coupler 100. The first laser system1000-1 further comprises a phase adjuster 1008 and a laser output 1012.In some embodiments, the first laser system 1000-1 comprises a detector1016.

The first mirror 1001 is optically coupled with the first input 104-1via a semiconductor waveguide (e.g., have a crystalline silicon core).The laser output 1012 is optically coupled with the second input 104-2via a waveguide. The detector 1016 is optically coupled with the secondoutput 108-2 via a waveguide. The second mirror 1002 is opticallycoupled with the gain medium 1004 via a waveguide (e.g., waveguidehaving a crystalline and/or silicon core). The gain medium 1004 isoptically coupled with the phase adjuster 1008 via a waveguide. Thephase adjuster 1008 is optically coupled with the first output 108-1 viaa waveguide.

The first mirror 1001 and the second mirror 1002 are binary supergratings (BSGs). Examples of BSGs, the gain medium 1004, and phaseadjuster 1008 are provided in commonly owned U.S. patent applicationSer. No. 13/605,633, filed on Sep. 6, 2012, which is incorporated byreference for all purposes. A BSG has a super period that defines areflectance spectrum. A reflectance spectrum has two or more reflectancepeaks, referred to as a super modes. In some embodiments a reflectancespectrum has between 3 and 12 super modes (e.g., 5, 7, 8, or 11).Cascading multiple super periods increases reflectance of the supermodes of the BSG. In some embodiments, the first mirror 1001 and thesecond mirror 1002 have a similar number of super periods, and/or thefirst mirror 1001 and the second mirror 1002 both have one or more supermodes having reflectance equal to or greater than 80%, 85%, 90%, 95%,97%, 98%, 99%, 99.5%, 99.9% or 100%. The first mirror 1001 and thesecond mirror 1002 form a resonator for the first laser system 1000-1.In some embodiments, super mode reflectance is equal to or less than100% for greater control when tuning to different frequencies. In anoptical path between the first mirror 1001 and the second mirror 1002,is the gain medium 1004, the phase adjuster 1008, and the directionalcoupler 100.

Some systems creates a laser resonator with two mirrors. Generally, oneof the two mirrors has a high reflectance (e.g., near 100%), and theother mirror has less reflectance to be used as an output coupler. Inthe first laser system 1000-1, the first mirror 1001 has a reflectancesimilar to the reflectance of the second mirror 1002. Output coupling ofthe first laser system 1000-1 is determined by the coupling efficiencyof the directional coupler 100 (and choosing inputs and outputs of thedirectional coupler 100) and not by a mirror being used as an outputcoupler. A system using BSGs will sometimes use one BSG (referred to asa “long BSG”) that has one or more super periods (often several) thananother BSG (referred to as a “short BSG”). more super periods one ormore super periods. Such a system is sometimes referred to as a“long-short BSG system.” In the long-short BSG system, the short BSG isused as an output coupler for the long-short BSG system because theshort BSG has less reflectance than the long BSG.

In some embodiments, the first mirror 1001 and the second mirror 1002are heated, thus shifting the reflectance spectrums of the BSGs. Thereflectance spectrum of the first mirror 1001 is different from thereflectance spectrum of the second mirror 1002. For example, spacingbetween reflectance peaks (“super modes”) of the first mirror 1001 isdifferent than spacing between super modes of the second mirror 1002.The first mirror 1001 and the second mirror 1002 are tuned (e.g., byheating) so that one super mode of the first mirror 1001 overlaps withone super mode of the second mirror 1002. By having higher BSGreflectance (e.g., cascading more super periods), spectral width of asuper mode is narrowed, giving more control for tuning the first lasersystem 1000-1 than if either the first mirror 1001 or the second mirror1002 was used as an output coupler and had fewer super periods.

The detector 1016 can be used for various purposes. In some embodiments,the detector 1016 is used for monitoring and control of laser power andfrequency of the first laser system 1000-1 by using a photodiode (PD)(e.g., a tap-PD). In some embodiments, the gain medium 1004 comprises aIII-V compound material and the first mirror 1001, the second mirror1002, the directional coupler 100, are made of silicon. In someembodiments, the first mirror 1001, the directional coupler 100, thephase adjuster 1008, the second mirror 1002, the detector 1016, and/orconnecting waveguides are formed together, monolithically, on asemiconductor chip (e.g., on an SOI wafer). In some embodiments, thedirectional coupler 100 is replaced with the third directional coupler700 to reduce potential reflections from the detector 1016 entering thelaser resonator.

Referring next to FIG. 11, a simplified diagram of a second laser system1000-2 is shown. The second laser system 1000-2 is similar to the firstlaser system 1000-1 except the first mirror 1001 is optically coupledwith the second input 104-2 and the laser output 1012 is opticallycoupled with the first input 104-1. The first mirror 1001 and the secondmirror 1002 of the second laser system 1000-2 have similar reflectivityfor a given wavelength when tuned. An equivalent reflectivity (R_(eq))is given by: R_(eq)=κ², where κ is the coupling efficiency of thedirectional coupler 100. Sometimes, κ<50% is considered weak couplingand κ≧50% is considered strong coupler. As an example, κ=30% (weakcoupling). Output coupling of the second laser system 1000-2(transmission) is given by: T (transmission)=1−κ=70%. And R_(eq)=κ²=9%.Thus 70% of power from the first output 108-1 (e.g., from a direction ofthe gain medium 1004) is transmitted to first input 104-1 and to thelaser output 1012; while 30% is transmitted to the second input 104-2and to the first mirror 1001. The first mirror 1001 reflects the 30%transmitted to the first mirror 1001 back to the second input 104-2.Thirty percent of the power reflected back to the second input 104-2 iscoupled into the first output 108-1 (30%×30%=9% of original power thatwas transmitted to the second output 108-1); and seventy percent of thepower reflected back to the second input 104-2 is coupled to the secondoutput 108-2 and transmitted to the detector 1016 (30%×70%=21% oforiginal power that was transmitted to the first output 108-1).

Referring back to FIG. 10, if κ=70% (strong coupling) for thedirectional coupler 100 of the first laser system 1000-1, then the firstlaser system 1000-1 would have the same effective reflectivity as thesecond laser system 1000-2 having a directional coupler 100 with κ=30%.

FIG. 12 depicts a chart comparing normalized combined responses of aY-branch laser and the laser system 1000 with the directional coupler100. The Y-branch laser is similar to a Y-branch laser in the commonlyowned '633 application. An x axis is frequency (THz) and a y axis isrelative power (dB). FIG. 12 shows that the laser system 1000 with thedirectional coupler 100 suppresses adjacent super modes more than theY-branch laser. In some embodiments, having suppressed adjacent supermodes allows the laser system 1000 to be more easily tuned.

FIG. 13 depicts a zoomed-in portion of the normalize combined responsesin FIG. 12. A combined response 1304, with a combined-response peak, forthe Y-branch laser is shown. The combined-response peak of the Y-branchlaser has a 1 dB bandwidth of about 40 GHz. A combined response 1308,with a combined-response peak, for the laser system 1000 is shown. Thelaser system 1000 with the directional coupler 100 has acombined-response peak having a 1 dB bandwidth of about 27 GHz. A firstlasing mode 1314 and a second lasing mode 1318 are shown. The firstlasing 1314 mode is for the Y-branch laser. The second lasing mode 1318is for the laser system 1000. A first side mode 1324 (e.g., cavitylongitudinal mode) and a second side mode 1328 (e.g., cavitylongitudinal mode) are shown. The first side mode 1324 is for theY-branch laser. The second side mode 1328 is for the laser system 1000.A first difference 1334 is shown. The first difference 1334 is a powerdifference between the first lasing mode 1314 and the first side mode1324. A second difference 1338 is shown. The second difference 1338 is apower difference between the second lasing mode 1318 and the second sidemode 1328. The second difference 1338 (˜2 dB) is about twice the firstdifference 1334 (˜1 dB). Thus the laser system 1000 will have betterside-mode suppression than the Y-branch laser. When lasing, this effectwill be exacerbated providing a side-mode suppression radio (SMSR)˜50 dBfor the laser system 1000 with the directional coupler 100.

The Y-branch laser is known to be better than the long-short BSG systemfor suppressing adjacent super modes, but can leave super modes that arefurther away (from a desired lasing frequency) with relatively highreflectivity. If the Y-branch laser has some asymmetry due to acentering offset (e.g., one BSG is not tuned with sufficient accuracy),the super modes that are further away can lase. On the other hand,long-short BSG systems are better than the Y-branch laser as suppressingsuper modes that are further away. One reason that long-short BSGsystems are worse at adjacent super-mode suppression is that the shortBSG of the long-short BSG system has wider, more rounded reflectionpeaks, which can overlap with one or more reflection peaks of the longBSG that are meant to be misaligned. In some embodiments, the lasersystem 1000 with the directional coupler 100 improves the best of boththe Y-branch laser and the long-short BSG system: the laser system 1000with the directional coupler has higher suppression of adjacent supermodes and super modes that are further away. The laser system 1000 alsohas higher suppression of adjacent cavity (longitudinal) modes than boththe Y-branch laser and the long-short BSG system.

FIG. 14 depicts a flowchart of an embodiment of a process 1400 for usingthe laser system 1000 with the directional coupler 100. The processbegins in step 1404 where a first wavelength selective element (e.g.,the first mirror 1001 using a first BSG) and a second wavelengthselective element (e.g., the second mirror 1002 using a second BSG) aretuned. The first wavelength selective element and the second wavelengthselective element form a laser resonator. In some embodiments, the firstwavelength selective element and the second wavelength selective elementare tuned using heat. In some embodiments, the first wavelengthselective element and the second wavelength selective element are tunedso that super modes of the first BSG and the second BSG have overlappingsuper modes at a predetermined frequency (e.g., tuning the laser system1000 using the Vernier effect).

In step 1408, light (optical emission) is generated by the gain medium1004 in the resonator (and/or reflected from the second mirror 1002). Insome embodiments, light is generated by applying current to a III-Vcompound (e.g., InP or GaAs) semiconductor structure having a quantumwell region. Light in the resonator is guided to the directional coupler100 (e.g., by a waveguide), step 1412. The directional coupler 100transmits a portion of the light out of the resonator, step 1416, andtransmits a portion of the light to the first wavelength selectiveelement (e.g., the first mirror 1001), step 1420.

Embodiments of the present invention provide a capability toindependently adjust output coupler transmission (and effective couplerreflectivity) of a laser from spectral properties of a mirror (e.g.,first mirror 1001). Thus some embodiments use long, high reflectivityBSGs, which have the advantages of: (1) narrower peaks, which can allowfor greater selectivity using the Vernier effect and suppression ofadjacent longitudinal modes; and (2) peaks of substantially uniformstrength; this is in contrast to lasers without a directional coupler100 that use short BSGs to achieve higher output coupler transmissionfor better slope efficiency. Also, some embodiments enable choosingvariable output coupling by designing an appropriate splitting ratio(e.g., coupling efficiency) of the directional coupler 100; whereaswithout the directional coupler 100, an ability to select outputcoupling of the laser (reflectivity and transmission) is coarse becausea length of a BSG is not continuously selectable, but typically selectedas an integral number of super periods.

Further, in some embodiments, it is desirable to have peak reflectanceof BSG super modes less than or equal to 100%. BSG length is long enoughto allow for high reflection in the center of the super modes, but nottoo long, in order to prevent a flat top of reflectivity at a givenfrequency. In some embodiments, curvature of a super mode is helpful toimprove suppression of adjacent cavity modes (longitudinal modes) withrespect to a lasing mode present in a center of an aligned pair of BSGs.In some embodiments, since: (1) it is desirable to have peak reflectanceof BSG super modes less than or equal to 100%; and (2) BSG reflectanceis increased by adding discrete number of super modes, the first mirror1001 (comprising a first BSG) and the second mirror 1002 (comprising asecond BSG) may have similar, but not equal reflectance peaks. Forexample, for a first given frequency, a reflectance peak of the firstBSG may be 82% while the second BSG reflectance peak may be 91%. Inanother example, for a given frequency, the first BSG may have areflectance peak at 99% and the second BSG have a reflectance peak at97%.

Referring next to FIG. 15, a top view of a fourth directional coupler1500 is shown. In some embodiments, the fourth directional coupler 1500,and/or variations on the fourth directional coupler 1500, are usedinstead of the first directional coupler 100. The fourth directionalcoupler 1500 is similar to the third directional coupler 600, but with awider shoulder 120. The first ridge 124-1 has a first outside edge1504-1, which is opposite an inside edge of the first ridge 1240-1 thatis closer to the longitudinal axis 128 than the first outside edge1504-1. The second ridge 124-2 has a second outside edge 1504-2, whichis opposite an inside edge of the second ridge 1240-2 that is closer tothe longitudinal axis 128 than the second outside edge 1504-2.

The fourth directional coupler 1500 has four regions: a first region1511, a second region 1512, a third region 1513, and a fourth region1514. In the first region 1511, the shoulder 120 tapers inward in alongitudinal direction, toward the longitudinal axis 128, as describedin connection to FIG. 3. Additionally, the shoulder 120 tapers outward,away from the first outside edge 1504-1 and the second outside edge1504-2. In some embodiments, the shoulder 120 tapers as much outward asthe shoulder 120 tapers inward. In the second region 1512 and the thirdregion 1513, the shoulder has a constant width (though in someembodiments, the shoulder could continue to taper and/or start totaper). In the fourth region 1514, the shoulder 120 tapers toward theoutside edges 1504, in addition to splitting as described in FIG. 3.

In some embodiments, the shoulder 120 tapers outside the outside edges1504 to reduce the coupling efficiency (i.e., reduce κ), and/or controlto more accuracy the coupling efficiency. In some embodiments, theshoulder tapers 120 outside the outside edges 1504 to reducepolarization rotation. Asymmetry about a ridge 124 can causepolarization rotation. For some applications, polarization rotation isundesirable. For example, in some Dense Wavelength Division Multiplexing(DWDM) systems, polarization must be very clear to separate differentpolarizations, and, in some DWDM systems, efficiency drops as even alittle polarization rotation is introduced.

The specific details of particular embodiments may be combined in anysuitable manner without departing from the spirit and scope ofembodiments of the invention. However, other embodiments of theinvention may be directed to specific embodiments relating to eachindividual aspect, or specific combinations of these individual aspects.

The above description of exemplary embodiments of the invention has beenpresented for the purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdescribed, and many modifications and variations are possible in lightof the teaching above. For example, in some embodiments, the directionalcoupler is disposed on a silicon substrate with other devices such as aCMOS device, a BiCMOS device, an NMOS device, a PMOS device, a detector,a CCD, diode, heating element, or a passive optical device (e.g., awaveguide, an optical grating, an optical splitter, an optical combiner,a wavelength multiplexer, a wavelength demultiplexer, an opticalpolarization rotator, an optical tap, a coupler for coupling a smallerwaveguide to a larger waveguide, a coupler for coupling a rectangularsilicon waveguide to an optical fiber waveguide, or a multimodeinterferometer).

The embodiments were chosen and described in order to explain theprinciples of the invention and practical applications to thereby enableothers skilled in the art to utilize the invention in variousembodiments and with various modifications as are suited to theparticular use contemplated.

Also, it is noted that the embodiments may be described as a processwhich is depicted as a flowchart, a flow diagram, a data flow diagram, astructure diagram, or a block diagram. Although a flowchart may describethe operations as a sequential process, many of the operations can beperformed in parallel or concurrently. In addition, the order of theoperations may be re-arranged. A process is terminated when itsoperations are completed, but could have additional steps not includedin the figure. A process may correspond to a method, a function, aprocedure, a subroutine, a subprogram, etc.

A recitation of “a”, “an”, or “the” is intended to mean “one or more”unless specifically indicated to the contrary.

All patents, patent applications, publications, and descriptionsmentioned here are incorporated by reference in their entirety for allpurposes. None is admitted to be prior art.

What is claimed is:
 1. A tunable laser comprising: a first wavelengthselective element characterized by a first reflectance spectrum; asecond wavelength selective element characterized by a secondreflectance spectrum, wherein the first wavelength selective element andthe second wavelength selective element form an optical resonator; again medium between the first wavelength selective element and thesecond wavelength selective element; and a directional coupler betweenthe first wavelength selective element and the second wavelengthselective element, wherein the directional coupler provides an outputcoupler for the laser.
 2. The tunable laser of claim 1, wherein: thefirst wavelength selective element is configured to have a firstreflectance value for a first reflectance peak; the second wavelengthselective element is configured to have a second reflectance value for asecond reflectance peak; and there is at least some overlap of betweenfrequencies in the first reflectance peak and the second reflectancepeak.
 3. The tunable laser of claim 2, wherein the first reflectancevalue and the second reflectance value are greater than 80%.
 4. Thetunable laser of claim 2, wherein: the first reflectance value and thesecond reflectance value are greater than 80%; and there is not morethan 10% difference in reflectance between the first reflectance valueand the second reflectance value.
 5. The tunable laser of claim 2,wherein: the first reflectance value and the second reflectance valueare greater than 90%; and there is not more than 5% reflectancedifference between the first reflectance value and the secondreflectance value.
 6. The tunable laser of claim 2, wherein the firstreflectance value and the second reflectance value are greater than 98%.7. The tunable laser of claim 6, wherein the first reflectance value andthe second reflectance value are greater than 99%.
 8. The tunable laserof claim 1, wherein the first wavelength selective element is a binarysuper grating.
 9. The tunable laser of claim 1, further comprising aphase adjuster that is configured to modify an optical path length ofthe optical resonator by heating a portion of a waveguide of the opticalresonator.
 10. The tunable laser of claim 1, further comprising anoptical sensor, wherein the optical sensor is optically coupled to thedirectional coupler.
 11. The tunable laser of claim 1, wherein the firstwavelength selective element, the second wavelength selective element,the gain medium, and the directional coupler are disposed on asubstrate.
 12. The tunable laser of claim 11, wherein: the firstwavelength selective element, the second wavelength selective element,the directional coupler comprise silicon; and the gain medium comprisesa III-V compound.
 13. The tunable laser of claim 1, further comprising aheating element coupled to the first wavelength selective element,wherein the first reflectance spectrum is adjustable as a function oftemperature of the heating element.
 14. The tunable laser of claim 1,wherein the directional coupler has a core that has a thickness between0.5 and 2.5 μm.
 15. A method of operating a tunable laser, the methodcomprising: tuning a first wavelength selective element; tuning a secondwavelength selective element, wherein the first wavelength selectiveelement and the second wavelength selective element form an opticalresonator; generating optical emission from a gain medium disposedbetween the first wavelength selective element and the second wavelengthselective element; guiding the optical emission to a directionalcoupler; transmitting a first portion of the optical emission out of theoptical resonator using the directional coupler; and transmitting asecond portion of the optical emission to the first wavelength selectiveelement using the directional coupler.
 16. The method of operating thetunable laser of claim 15, wherein: the first wavelength selectiveelement comprises a binary super grating (BSG) having a first number ofsuper periods; the second wavelength selective element comprises a BSGhaving a second number of super periods; and the first number of superperiods equals the second number of super periods.
 17. The method ofoperating the tunable laser of claim 15, wherein the first wavelengthselective element is tuned by changing a temperature of a heatingelement attached to the first wavelength selective element.
 18. Themethod of operating the tunable laser of claim 15, wherein: the firstwavelength selective element comprises a binary super grating with areflectance value greater than 98%; and the second wavelength selectiveelement comprises a binary super grating with a reflectance valuegreater than 98%.
 19. The method of operating the tunable laser of claim15, wherein: the first wavelength selective element, the secondwavelength selective element, the directional coupler comprise silicon;and the gain medium comprises a III-V compound.
 20. The method ofoperating the tunable laser of claim 15, further comprising transmittinga third portion of the optical emission to an optical sensor.